Acid-Base Disorders Classification

1. ICU Book 4th Edition 腎臟科 巫文平 醫師 2015/4/23

2. Classification of Acid-Base Disorders • [H+] = 24 × (PCO2/HCO3)

3. Secondary Responses • Acute respiratory alkalosis: Expected HCO3 = 24 – [0.2 × (40 – current PaCO2)] • Chronic respiratory alkalosis:Expected HCO3 = 24 – [0.4 × (40 – current PaCO2)]

4. Metabolic Acidosis • EXAMPLE: For a metabolic acidosis with a plasma HCO3 of 14 mEq/L, • ΔHCO3 is 24 – 14 = 10 mEq/L, • ΔPaCO2 is 1.2 × 10 = 12 mm Hg, • Expected PaCO2 is 40 – 12 = 28 mm Hg. • If the PaCO2 is > 28 mm Hg, there is a secondary respiratory acidosis. • If the PaCO2 is < 28 mm Hg, there is a secondary respiratory alkalosis. Expected PaCO2 = 40 – [1.2 × (24 – current HCO3)]

5. Metabolic Alkalosis • EXAMPLE: For a metabolic alkalosis with a plasma HCO3 of 40 mEq/L, • ΔHCO3 is 40 – 24 = 16 mEq/L • ΔPaCO2 is 0.7 × 16 = 11 mm Hg • Expected PaCO2 is 40 + 11 = 51 mm Hg Expected PaCO2 = 40 + [0.7 × (current HCO3 – 24)]

6. Acute Respiratory Disorders • EXAMPLE: For an acute increase in PaCO2 to 60 mm Hg, • ΔHCO3 is 0.1 × 20 = 2 mEq/L for an acute respiratory acidosis • The expected HCO3 is 24 + 2 = 26 mEq/L Acute respiratory acidosis Expected HCO3 = 24 + [0.1 × (current PaCO2 – 40)] Acute respiratory alkalosis Expected HCO3 = 24 – [0.2 × (40 – current PaCO2)]

7. Chronic Respiratory Disorders • EXAMPLE: For an increase in PaCO2 to 60 mm Hg that persists for at least a few days • ΔPaCO2 is 60 – 40 = 20 mm Hg, • ΔHCO3 is 0.4 × 20 = 8 mEq/L, • The expected HCO3 is 24 + 8 = 32 mEq/L. Chronic respiratory acidosis Expected HCO3 = 24 + [0.4 × (current PaCO2 – 40)] Chronic respiratory alkalosis Expected HCO3 = 24 – [0.4 × (40 – current PaCO2)]

8. STEPWISE APPROACH TO ACID-BASE ANALYSISStage I: Identify the Primary Acid-Base Disorder Rule 1: If the PaCO2 and/or the pH is outside the normal range, there is an acid-base disorder. Rule 2: If the PaCO2and pH are both abnormal, compare the directional change. 2a. If the PaCO2 and pH change in the same direction, there is a primary metabolic acid-base disorder. 2b. If the PaCO2 and pH change in opposite directions, there is a primary respiratory acid-base disorder.

9. EXAMPLE-Rule 2 • a case where the arterial pH = 7.23 and the PaCO2 = 23 mm Hg. • The pH and PaCO2 are both reduced (indicating a primary metabolic disorder) • The pH is low (indicating an acidosis), so the diagnosis is a primary metabolic acidosis.

10. Stage I: Identify the Primary Acid-Base Disorder, Rule 3 Rule 3: If only the pH or PaCO2 is abnormal, the condition is a mixed metabolic and respiratory disorder (i.e., equal and opposite disorders). 3a. If the PaCO2 is abnormal, the directional change in PaCO2 identifies the type of respiratory disorder (e.g., high PaCO2 indicates a respiratory acidosis), and the opposing metabolic disorder. 3b. If the pH is abnormal, the directional change in pH identifies the type of metabolic disorder (e.g., low pH indicates a metabolic acidosis) and the opposing respiratory disorder.

11. EXAMPLE-Rule 3 •  A case where the arterial pH = 7.38 and the PaCO2 = 55 mm Hg. • Only the PaCO2 is abnormal, so there is a mixed metabolic and respiratory disorder • The PaCO2 is elevated, indicating a respiratory acidosis • The metabolic disorder must be a metabolic alkalosis. • Therefore, this condition is a mixed respiratory acidosis and metabolic alkalosis. • Both disorders are equivalent in severity because the pH is normal.

12. Stage II: Evaluate the Secondary Responses- Metabolic Rule 4: (Metabolic) For a primary metabolic disorder, if the measured PaCO2 is higher than expected, there is a secondary respiratory acidosis, and if the measured PaCO2 is less than expected, there is a secondary respiratory alkalosis. • EXAMPLE: a case where the PaCO2 = 23 mm Hg, the pH = 7.32, and the HCO3 = 16 mEq/L. • The pH and PCO2 change in the same direction, indicating a primary metabolic disorder • The pH is acidemic, so the disorder is a primary metabolic acidosis. • The ΔPaCO2 is 1.2 × (24 – 16) = 10 mm Hg (rounded off), and the expected PaCO2 is 40 – 10 = 30 mm Hg. • The measured PaCO2 (23 mm Hg) is lower than the expected PaCO2, so there is an additional respiratory alkalosis. • Primary metabolic acidosis with a secondary respiratory alkalosis.

13. Stage II: Evaluate the Secondary Responses-Respiratory Rule 5: For a primary respiratory disorder, a normal or near-normal HCO3 indicates that the disorder is acute. Rule 6: For a primary respiratory disorder where the HCO3 is abnormal, determine the expected HCO3 for a chronic respiratory disorder. 6a. For a chronic respiratory acidosis, if the HCO3 is lower than expected, there is an incomplete renal response, and if the HCO3 is higher than expected, there is a secondary metabolic alkalosis. 6b. For a chronic respiratory alkalosis, if the HCO3 is higher than expected, there is an incomplete renal response, and if the HCO3 is lower than expected, there is a secondary metabolic acidosis.

14. EXAMPLE: Rule 5-6 • A case where the PaCO2 = 23 mm Hg, the pH = 7.54, and the HCO3 = 17 mEq/L. • The PaCO2 and pH change in opposite directions, and the pH is alkaline • Primary respiratory alkalosis. • The HCO3 is abnormal-> not an acute respiratory alkalosis. • ΔPCO2 is 40 – 23 = 17 mm Hg, the ΔHCO3 is 0.4 × 17 = 7 mEq/L, and the expected HCO3 is 24 − 7 = 17 mEq/L. • The measured HCO3 is the same as expected for a chronic respiratory alkalosis, so this condition is a chronic respiratory alkalosis with an appropriate (completed) renal response. • If the measured HCO3was higher than 17 mEq/L (but < 21 • Chronic respiratory alkalosis with an incomplete renal response • if the measured HCO3 was lower than 17 mEq/L • Chronic respiratory alkalosis with a secondary metabolic acidosis.

15. Stage III: Use The “Gaps” to Evaluate a Metabolic Acidosis • The Anion Gap • The Gap-Gap Ratio

16. Anion gap: AG = Na – (CL + HCO3)(reference range for the AG was 12±4 mEq/L) • The anion gap is a rough estimate of the relative abundance of unmeasured anions, and is used to determine if a metabolic acidosis is due to an accumulation of non-volatile acids (e.g., lactic acid) or a primary loss of bicarbonate (e.g., diarrhea)

17. Influence of Albumin • AGc = AG + 2.5 × (4.5 – [albumin in g/dL]) • albumin is the principal unmeasured anion, and the principal determinant of the anion gap • A low albumin level in plasma will lower the AG, and this could mask the presence of an unmeasured anion

19. The Gap-Gap Ratio • AG Excess/HCO3 Deficit = (AG – 12)/24 – HCO3) • High AG metabolic acidosis • Gap-gap ratio = 1: High AG metabolic acidosis, the decrease in serum HCO3 is equivalent to the increase in AG • Gap-gap ratio < 1: Co-existence of a normal AG (hyperchloremic) metabolic acidosis • Gap-gap ratio > 1: Co-existence of a metabolic alkalosis.

20. Summary • Identify the Primary Acid-Base Disorder • Rule 1-3 • Evaluate the Secondary Responses • Rule 4-6 • Use The “Gaps” to Evaluate a Metabolic Acidosis • The Anion Gap • AG = Na – (CL + HCO3) • Influence of Albumin on AG • AGc = AG + 2.5 × (4.5 – [albumin in g/dL]) • The Gap-Gap Ratio • AG Excess/HCO3 Deficit = (AG – 12)/24 – HCO3)

21. Secondary Responses • Acute respiratory alkalosis: Expected HCO3 = 24 – [0.2 × (40 – current PaCO2)] • Chronic respiratory alkalosis:Expected HCO3 = 24 – [0.4 × (40 – current PaCO2)]